As illustrated in FIG. 1, a portion of a conventional inkjet printer 90 includes a printer carriage 92 and a print cartridge 94 installed in the printer carriage. The print cartridge includes a printhead which ejects or fires ink drops 96 through a plurality of orifices or nozzles 95 and toward a print medium 98, such as a sheet of paper, so as to print a dot of ink on the print medium. Typically, the orifices are arranged in one or more columns or arrays such that properly sequenced ejection of ink from the orifices causes characters or other images to be printed upon the print medium as the print cartridge and the print medium are moved relative to each other.
Image quality and performance of inkjet printing is rapidly approaching that of silver halide photographs and offset printing. The greatest improvement in image quality has been achieved by increasing image resolution which is a measure of the number of dots printed per height of an image, for example, dots-per-inch. Image resolution has been increased by reducing orifice spacing of the printhead and reducing a volume of the ink drops with an understanding that the volume of an ink drop corresponds to a size of the dot formed on the print medium. By reducing the orifice spacing of the printhead and the size of the ink drops, an image becomes sharper, less grainy, and more detailed.
As orifice spacing and drop volume decrease to increase image resolution, however, it becomes necessary to operate the printhead at higher firing frequencies and faster printing speeds to achieve the same throughput. Unfortunately, smaller, more closely spaced ink drops ejected at higher firing frequencies are more greatly influenced by surrounding air than larger, more widely spaced ink drops ejected at lower firing frequencies. Analysis has shown that the rate of kinetic energy transfer between an ink drop and the surrounding air is proportional to the surface area of the ink drop. The kinetic energy transfer rate of many small drops, therefore, is greater than that of fewer large drops. This kinetic energy transfer phenomena generates air currents which develop into air vortices formed between nozzle columns of the printhead. Examples of such air currents and formed air vortices are indicated at 99 in FIG. 1.
The air currents and air vortices, however, misdirect the ink drops as they are ejected toward the print medium and through a print zone. Unfortunately, misdirection of the ink drops yields images which have undesirable print defects or artifacts, including banding and/or “worms.” Banding is more prominent in medium density area fills, such as graphics and images, and is characterized by random light and dark bands across an image. Banding is typically caused by misdirection of the ink drops in a paper axis (i.e., a direction perpendicular to a scanning axis). The dark bands result when misdirected ink drops land on ink drops ejected from adjacent nozzles of the printhead and the light bands represent uncovered areas or white space resulting from the same misdirected ink drops. Banding is readily detected at normal viewing distances and is typically very objectionable to a viewer.
Worms are also more prominent in medium density graphics and are characterized by a mottled appearance of an image. Worms are typically caused by a localized misdirection of the ink drops. A predominate cause of worms in low drop volume printheads is misdirection of the ink drops due to air currents generated by air entrained by the ink drops as the ink drops are ejected through the print zone. As such, these air currents disrupt and misdirect trajectories of the ink drops yielding areas of non-uniform area fill, hue shifts, and poor image resolution.
Attempts to mask or hide these print defects have utilized multi-pass print modes, reduced printing speeds, and/or reduced spacing between the print cartridge and the print medium (i.e., pen-to-paper spacing). These attempts, however, are leading in a direction contrary to the desired direction of inkjet printer advancement, such as single-pass print modes, faster printing speeds for higher throughput, increased pen-to-paper spacing for accommodating a greater range of print medium thickness, and higher resolution, lower drop volume printheads.
Accordingly, a need exists for an inkjet printer which substantially eliminates objectionable print defects, such as banding and/or worms, caused by air currents generated by printing, without compromising image resolution, printing speed, and/or print medium flexibility.